[EventEngine] Returns error instead of assertion when hostname is empty (#35306)

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Closes #35306

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35306 from yijiem:dns-empty-hostname 9c7f67c27a
PiperOrigin-RevId: 591028370

src/core/lib/event_engine/ares_resolver.cc

@@ -241,7 +241,13 @@ void AresResolver::LookupHostname(
              "Unparseable name: ", name))]() mutable { callback(status); });
     return;
   }
-  GPR_ASSERT(!host.empty());
+  if (host.empty()) {
+    event_engine_->Run([callback = std::move(callback),
+                        status = absl::InvalidArgumentError(absl::StrCat(
+                            "host must not be empty in name: ",
+                            name))]() mutable { callback(status); });
+    return;
+  }
   if (port_string.empty()) {
     if (default_port.empty()) {
       event_engine_->Run([callback = std::move(callback),
@@ -307,7 +313,13 @@ void AresResolver::LookupSRV(
              "Unparseable name: ", name))]() mutable { callback(status); });
     return;
   }
-  GPR_ASSERT(!host.empty());
+  if (host.empty()) {
+    event_engine_->Run([callback = std::move(callback),
+                        status = absl::InvalidArgumentError(absl::StrCat(
+                            "host must not be empty in name: ",
+                            name))]() mutable { callback(status); });
+    return;
+  }
   // Don't query for SRV records if the target is "localhost"
   if (absl::EqualsIgnoreCase(host, "localhost")) {
     event_engine_->Run([callback = std::move(callback)]() mutable {
@@ -336,7 +348,13 @@ void AresResolver::LookupTXT(
              "Unparseable name: ", name))]() mutable { callback(status); });
     return;
   }
-  GPR_ASSERT(!host.empty());
+  if (host.empty()) {
+    event_engine_->Run([callback = std::move(callback),
+                        status = absl::InvalidArgumentError(absl::StrCat(
+                            "host must not be empty in name: ",
+                            name))]() mutable { callback(status); });
+    return;
+  }
   // Don't query for TXT records if the target is "localhost"
   if (absl::EqualsIgnoreCase(host, "localhost")) {
     event_engine_->Run([callback = std::move(callback)]() mutable {

test/core/event_engine/test_suite/tests/dns_test.cc

@@ -591,4 +591,9 @@ TEST_F(EventEngineDNSTest, UnparseableHostPortsBadLocalhostWithPort) {
   TestUnparseableHostPort(CreateDNSResolverWithoutSpecifyingServer(),
                           &dns_resolver_signal_, "[localhost]:1");
 }
+
+TEST_F(EventEngineDNSTest, UnparseableHostPortsEmptyHostname) {
+  TestUnparseableHostPort(CreateDNSResolverWithoutSpecifyingServer(),
+                          &dns_resolver_signal_, ":443");
+}
 // END